Purpose:To demonstrate an increase spin-spin relaxation time (T2) of hyperpolarized carbon-13 urea by nitrogen-15 labeling its amide groups, and to show how this enables background-free perfusion MRI at 1 mm in-plane resolution with SNR > 300.

Results:Measured T2 values were 90±15 ms for [C13]urea and 24±4 s for [C13,N-15]urea; the in vivo estimate of the T2 of [C13,N-15]urea was 1.4 s. Concordant with analytic signal models, hyperpolarized imaging with [C-13,N-15] urea showed SNR improvements of 5.1±2.3 and 3.5±1.1 compared to [C13]urea imaged with FLASH and SSFP respectively. Exponential decay maps showed delineation of the renal cortex, medulla, and pelvis, with renal columns (medullary cortical extensions) visible.

Conclusion:The large SNR gain achievable using [C13,N-15]urea could be utilized for a corresponding resolution enhancement, which greatly improves the image quality when using this completely endogenous probe. This probes low toxicity could enable contrast angiography on patients with diminished renal function, even with large intravenous doses. Since clinical MRI scanners have longer minimum repetition time requirements due to safety and hardware constraints, the long T2 of [C13,N-15]urea measured in this study facilitates SSFP or spin echo train imaging of hyperpolarized for clinical applications. The delineation of the renal structures suggests a possible application of renal function assessment.